Abstract: Dileptons provide direct observables of the electromagnetic current-currentcorrelator in the hot and-or dense medium formed in heavy-ion collisions. Inthis article an overview is given about the status of the theoreticalunderstanding of the dilepton phenomenology in heavy-ion collisions fromstudies of the in-medium properties of hadrons and partons within many-bodytheory and about connections to fundamental questions concerning the chiralphase transition.